Brown: APOGAMY IN PHEGOPTERIS POLYPODIOIDES 21 
In the case of the cultures which were kept in the greenhouse, 
the majority of the prothallia growing on the modified Prantl’s 
and Knop’s solutions did not develop beyond a filamentous 
stage and lived only from five to six weeks. This behavior was 
attributed to temperature rather than light: the temperature 
in the greenhouse being very high during the greater part of 
the day, the culture media became overheated, with the result 
that the prothallia, lacking in vigor on account of an insufficient 
supply of some essential element in the nutrient solutioa, were 
unable to withstand the unfavorable conditions of temperature. 
A few prothallia in the greenhouse cultures on Prantl’s and 
Knop’s unmodified solutions survived, however, and developed 
into normal heart-shaped thalli. 
The length attained by the prothallium during its filamentous 
stage varied greatly, but even in cultures on the full nutrient 
solutions it was not uncommon to find prothallia with filaments 
of seven and eight cells in length. These usually occurred in 
crowded regions of the cultures. In the cultures on the modified 
solutions the length of the filaments was even greater, some having 
as many as twelve to fifteen cells. The length of the cells also 
varied, some being long and others short, but with the long 
cells predominating. It is customary for the length of the 
filament of the prothallia of any species of ferns to vary under 
unfavorable environmental conditions, but the range of varia- 
bility in length seems fixed for each species. In this respect 
Phegopteris polypodioides showed the greatest range of varia- 
bility of any prothallia of the Polypodiaceae that have come 
under the author’s observation. 
Branching was exceedingly varied and occurred frequently in 
the cultures on the modified solutions, while on the unmodified 
solutions it was not uncommon. Many times the first cell of 
the filament, instead of dividing transversely, divided length- 
wise, giving rise to two distinct filaments, which later broadened 
into prothallia. Sometimes other cells of the filaments divided, 
giving rise to branches which in turn broadened into prothallia. 
In other cases a filament of three cells was first formed, and then 
the first and second cells of the filament divided lengthwise, 
giving rise to side branches. These side branches continued 
their growth as filaments for a time, and then branched dichoto- 
mously. In still other cases the end cell of a filament of four 
